The ultimate goal in design of a guitar neck is to attain and maintain balance on the fine line of playability, without sacrificing sound quality. Playability can be defined as the distance, and thus the finger force, necessary to play (or fret) a note clearly. Of course, the most desired case is the smallest possible string-to-fret distance at all frets along the entire span of the neck.
In attempting to obtain this optimum distance, several challenges have long plagued the industry. A prominent one of these is the annoying fret buzz sound caused by the string slapping against a higher fret as it vibrates in its elliptical path. Depending on the intensity and the direction at which the string is plucked, the amplitude of the string in the direction of the fret will vary. Since each musician's playing technique is different, an ability to adjust the neck is required in order to eliminate buzz and achieve other benefits.
Fret buzz (and related problems) cannot be effectively eliminated in the manufacturing process by simply sanding down the deviant fret until the buzz subsides. Although this is done routinely as a standard part of the manufacturing process, it is a temporary fix. The fix is temporary due to the ever changing forces acting on the neck, and the resulting displacements that occur subsequent to manufacture.
Necks made primarily of wood are desired by a great many guitarists. But the design of wooden necks involves several pitfalls inherent in working with a substance produced by nature. Wood varies in properties from piece to piece. Each log is different, and it is not practical to cut each neck (wood block for a neck) from exactly the same location along the log. There are an infinite number of possible grain angles, distances from center, etc.
Forces acting on the neck, subsequent to manufacture, include varying string-tension forces and further include forces due to expansion and contraction of the wood in response to varying moisture content. Relative to the latter, the cells in the wood exchange moisture with the air until equalization occurs, and expand and contract in so doing. This causes a neck to move significantly, particularly in the case of musicians who travel to different climates over short periods of time.
The conventional way to adjust a guitar neck, to eliminate the results of the above-specified and other forces, is to use a truss rod. Truss rods have existed for decades, but have certain points of ineffectiveness. Typical rods act primarily on the basis of reverse tensioning in order to counteract string forces. However, it is the opinion of applicant that these rods cause unwanted humps along the fingerboard when certain adjustments occur. Such humps are highly undesired because they require an increase in over-all distance between frets and strings in order to prevent fret buzz.